Understanding how IL-10, released by various immune cells in the gastrointestinal tract, affects the self-renewal of ISCs and the balance and homeostasis of intestinal epithelial proliferation and differentiation will help to understand the initiation, progress, and treatment of various gastrointestinal diseases, in which IL-10 is involved. In this study, we firstly demonstrated that IL-10 reduces the survival of Lgr5+ ISCs and proliferation of epithelial cells by inhibiting Notch and Wnt signaling, but promotes enhanced the differentiation of epithelial cells and expanded numbers of TA cells. In addition, this is novel finding that increased expression of Paneth cells and decreased expression of enteroendocrine cells and goblet cells were induced by rmIL-10. These suggest that IL-10 damages the self-renewal of ISCs, which is not conducive to the repair of intestinal mucosal injury.
Intestinal inflammation plays an important role in the repair and damage of intestinal mucosa, and IL-10, as a factor released directly by immune cells, plays its role in the direct interaction between immune cells and intestinal epithelium (17). IL-10 from different cells has been shown to perform multiple functions in a variety of intestinal diseases. In IBD, IL-10 signaling in macrophages, rather than in T cells, is essential for inducing CD206+ regulatory macrophages and anti-TNF therapeutic responses (11). Consistent with these results, it has been confirmed that intestinal IFN-γ-producing type-1 regulatory T cells selectively down-regulated IL-10 in response to pro-inflammatory cytokines and may cause excessive intestinal inflammation in patients with IBD (18). In addition, this study showed that resident gut bacteria activated IL-10-producing B cells, which reduced colon T cell activation and maintained mucosal homeostasis to the gut bacteria (12). Short-chain fatty acids derived from microbiota can promote the production of IL-10 in Th1 cells, thereby maintaining intestinal homeostasis (10). Zhang et al. revealed that IL-10 could be a potential target for the damages of intestinal barrier and immunity in intestinal ischemia reperfusion (19). Furthermore, restoring normal bacteria through fecal microbiota transplantation (FMT) is considered a promising treatment for many diseases, and FMT activates immune cells to release IL-10 in order to suppress intestinal inflammation (20). Finally, bone marrow transplantation performed 24 h after total body irradiation improves intestinal barrier and stem cell recovery with induction of IL-10 and Notch signaling (21). All these indicate that IL-10 released by different immune cells plays an important role in maintaining intestinal homeostasis and anti-inflammatory. However, in this study, we firstly uncovered that IL-10 may play an adverse role in the process of intestinal mucosa repair and self-renewal of ISCs.
A small group of Lgr5 ISCs divide regularly to produce highly proliferating progenitor cells, called TA cells, which eventually differentiate into an absorbable (enterocytes) or secretory (Paneth cells, goblet cells and enteroendocrine cells) cell lineage, while gradually moving up to the top of the villi (22). The close interaction between Lgr5 ISCs and Paneth cells is critical to maintaining the fate of Lgr5 ISCs, because Paneth cells provide crucial niche factors for self-renewal and differentiation of Lgr5 ISCs (such as Wnt3a, BMP and Notch factors) (23). Paneth cells express Notch ligands, which bind to the Notch receptor on the Lgr5 ISCs to activate expression of downstream genes such as Hes1 and Hes5. ISCs proliferation and maintenance are controlled by the Wnt/β-catenin signaling pathway. Wnt ligands are released by Paneth cells and stromal cells (including macrophages, endothelial cells and neurons) located below the epithelial surface. Once released, Wnt ligands activate the low-density lipoprotein receptor-related protein 5/6 (Lrp5/6) and the frizzled protein co-receptors to facilitate β-catenin transport to the nucleus where it interacts with transcription factor 4 to maintain proliferation and differentiation of stem cells and epithelial cells. Here, we firstly demonstrated that IL-10 reduced ISCs survival and epithelial proliferation, and suppressed Notch and Wnt signaling. However, the number of Paneth cells was increased in IL-10 treated organoids and mice.
This study still has many limitations. This study only observed the effects of changes in IL-10 levels on intestinal epithelium cells and ISCs under normal conditions. However, we did not establish a specific disease model to observe the effects of changes in IL-10 levels on intestinal epithelium cells and ISCs. Therefore, the conclusion of this study may not be applicable to certain disease models. In addition, the signaling pathways that promote self-renewal of ISCs are not only Wnt and Notch, but also signaling pathways such as BMP. In this study, we did not observe the effect of IL-10 on other signaling pathways other than Wnt and Notch that promote self-renewal of ISCs. Meanwhile, we did not observe the effect of IL-10, an anti-inflammatory factor, on the inflammation level of ISCs and epithelial cells, and the number of macrophages and neutrophils etc.. Overall, we will establish a specific research model in future research to further explore the possibility and mechanism of IL-10 as a potential therapeutic target.